1. Field of Invention
The present invention relates to pattern recognition systems and more particularly to apparatus for identifying and removing backgrounds from a video image of a moving target in an optical tracking system.
2. Description of the Prior Art
It is well known in the prior art to track moving objects sush as aircraft, vechicles, and the like by means of optical systems. Commonly, a video camera, which may be sensitive to visible light or infrared energy, will be aimed at a scene containing the object to be tracked. The scene may be displayed on a cathode ray tube. The video signal from the camera may be processed to produce an electronic tracking gate which controls a servo mechanism system coupled to the camera platform. The electronic tracking circuits thereby control the platform position and attitude to maintain the targets centered within the electronic gate. The display system permits initial manual tracking of the target prior to switching to the automatic mode. Such systems have many uses including tracking a target prior to launching of a missile or the like; for use within a missile for tracking and guidance of the missile into a target; and for controlling missiles after launch from the launch vehicle or from a ground location. In noncombat applications, tracking systems of this type may be used during testing of missiles, vehicles, aircraft and the like to provide bearing and velocity information to a ground monitoring station.
In all of these applications of optical tracking systems, one inherent problem obtains from variations in the backgound of the target. For example, as the object being tracked moves from a dark background, such as would be typical of trees, into a very light area such as a sky or clouds, the sharp changes in contrast in the video scene may be interpreted by the tracking system as a target movement causing the tracking window to slew off and lose lock. This action often results in complete loss of target with resultant failure of the mission. Thus, there is a need for means to discriminate between the target and the background in video tracking systems. For example, if the background portion of the video pattern may be recognized and identified as background, all video information from such background may be deleted from the electronic tracking circuits. Therefore, abrupt changes and contrasts of background would not interfere with the tracking process.
In U.S. Pat. No. 3,444,380, Webb teaches a method and apparatus for suppressing the background in a field scanning sensor. The method utilizes digital space correlation to suppress false target signals as applied to a point target tracking device. The search field of a tracking system is divided into a matrix of rows and columns of elemental fields of view. Each elemental field view is examined to determine if a target exists in that field and matrix neighbors are compared to determine if target signal exist in adjacent elemental fields. The system rejects a signal if its adjacent matrix neighbor contains a signal.
U.S. Pat. No. 3,586,770 to Bonebreak et al discloses a system for distinguishing a specified object in space from objects of similar intensity within a receiving aperture of a television camera. In this patent, a video processor is coupled to the television camera and limits the system response to signals representative of the inner intensity contour of possible targets. A digital processor responds to the video processor output signals to determine the difference between the angular location of the designated object and a previously stored estimate of this position and then updates the stored position. The updating function is normalized by the target image dimension so that the tracking response of the system is essentially independent of the target image size. The video processor unit eliminates signals which are not representative of a designated target on the basis of signal amplitude comparisons. Digital logic circuits distinguish between the design target and false targets on the basis of angular location.
Neither of these prior art approaches provides an effective and efficient means for removing a background signal from a video image containing a moving target. The use of amplitude discrimination results in loss of sensitivity in the presence of noise.